Metallothioneins in the clam Ruditapes decussatus: an overview

coastal waters. Due to its economic importance it is extensively cultured in many countries, and particularly in Portugal where its production represents > 80 % of total shellfish production. The selection of the clam R. decussatus as a bioindicator was a consequence of its economic importance and the need to use a filter-feeding bivalve to evaluate environmental changes of metal concentrations [1]. Like many other bivalves, this species resides in sediments and accumulates metal concentrations reflecting gradients of metal contamination in the surrounding environment [2,3,4]. This suggests that its tissues have mechanisms, related to its filter-feeding habit, that inhibit the toxic effects of these contaminants. Like many other molluscs R. decussatus has evolved a number of subcellular systems for accumulation, regulation and immobilization of metals. These include, among others, the binding of essential and pollutant metals to soluble ligands such as MT. These two-domain molecules are low molecular, heat-stable proteins of non-enzymatic nature that occur mainly in the cytoplasm. They have strong affinity to class B metal cations which enable them to be differentiated from most of the other proteins [5]. The induction of these proteins has, therefore, been proposed as a specific indicator and possible “early warning marker” for the detection of detrimental effects caused by exposure to excess of essential and pollutant metals. MT induction has been detected in the whole soft tissues, gills, digestive gland and remaining tissues of the clam R. decussatus after exposure to essential and toxic metals [4,6] and this paper represents an overview of MT function in this bivalve species. MT induction in laboratory experiments